abnormal buzz in metro 100 watt plexi

dast · Post by **dast** » Fri Dec 21, 2007 5:02 pm

hello,

I have what I consider to be an extra loud buzz in my new build. It is absent at standby, but present when standby is on and volumes are down. It gets larger when volumes are up. The amp is certainly playable - but I do believe the buzz to be more than normal. Especially since some claim their metro builds are dead quiet.

I have a ground bus for the pots, and aside from that, the filter caps and resistors on power tubes are being grounded to chassis. My filament twists are good.

Does anyone have any suggestions? I read one post where someone moved the ground on the filter cap under the circuit board and that solved his problems, but I can't locate that post.

Thanks,
Dave

toner · Post by **toner** » Fri Dec 21, 2007 5:28 pm

some quick suggestions:

swap preamp tubes

check for cold solder joints

input jack wiring
- keep it as short as possible and check the grounds

preamp socket lead dress
- lift grid and cathode wires up away from the chassis and keep them away from plate wires
- keep all wires away from the heater wires

Try the amp in a different room or building to make sure it's the amp and not AC noise.

gutpile · Post by **gutpile** » Fri Dec 21, 2007 9:48 pm

is it buzzing in the head case or out? buzzing will sometimes be louder outside of the head case because it does not have the benefit of the shielding inside the head case....

Gibanez · Post by **Gibanez** » Fri Jan 04, 2008 9:22 pm

Toner, I had a real bad buzz in my first PTP in a SLP reissue.
When I put the new board in, I grounded the cathode to a lug
I put in under one of the standoffs and it was buzzing like crazy.
I then grounded the cathode to the input jack like Marshall had
before I took out the PC board and the buzz was gone. I still
can't figure out why I had the noise when it was grounded to
the lug

gtomax · Post by **gtomax** » Mon Jan 07, 2008 5:42 pm

Should the pot buss be grounded to the chassis?

dast · Post by **dast** » Sun Jan 13, 2008 6:03 pm

good question - I believe I've grounded the pot bus to chassis in 2 locations. Hopefully this is my problem. Does anyone have an opinion where the best ground for pot bus is?

I seem to have it grounded to the filter cap under the board and also one of the grouped filter caps.

any suggestions?
thanks.
Dave

dast · Post by **dast** » Sun Jan 13, 2008 6:26 pm

I also have the cathode resistor and cap going to the ground bus. Perhaps it would be better to remove the under board cap from ground buss and route it to this board connection?
thanks,
Dave

gtomax · Post by **gtomax** » Mon Jan 14, 2008 12:03 pm

Here's how I have mine: Power filter caps, PI filter cap and PI pot to one point (same as where HT fuse goes to chassis). Inputs to their own point on chassis with short lead. Pot buss to input grounds.

dast · Post by **dast** » Mon Jan 14, 2008 2:55 pm

hi,
when you say PI cap - is this the under board filter capacitor? And does PI stand for phase invertor? Which pot would PI pot be?
thanks for your help,
Dave

gtomax · Post by **gtomax** » Mon Jan 14, 2008 3:02 pm

Whoops, I mean presence pot... sorry

PI stands for phase inverter, yes. The PI filter cap on mine is on the power supply filter board as I built a JTM100 but on a later 100w it should be the can cap next to the output tubes. Also called V3 filter on the Batz layouts. It is the one connected to one side of the 10k2w resistor.

William Payne · Post by **William Payne** » Tue Jan 15, 2008 1:22 am

also try checking the guitar lead , I may not be able to play guitar good yet but one thing ive noticed is a crappy lead can make an anoying buzz

Metropoulos Forum

abnormal buzz in metro 100 watt plexi

abnormal buzz in metro 100 watt plexi

where should pot buss be grounded to chassis

cathode resistor and cap also goes to pot ground buss

thanks - and a follow up question